Containers which were conventional in the past and which made of white or colored sheet metal, glass or also ceramic are being increasingly displaced by plastic containers. Mainly plastic containers are used for packaging of fluid substances, for example beverages, oil, cleaning agents, cosmetics, etc. The low weight and lower costs play a not insignificant part in this substitution. The use of recyclable plastic materials and overall more favorable total energy balance in their production also contribute to promoting the acceptance of plastic containers, especially plastic bottles, among consumers.
The production of plastic containers, especially plastic bottles, for example from polyethylene or polypropylene, takes place in an extrusion blowing process, especially in a process of blowing extruded tubes. In doing so a plastic tube is extruded from an extruder head, placed in blow molding tools, inflated by way of a blowing mandrel by overpressure, and hardened by cooling. The extrusion blowing machines used for this purpose generally have at least one extruder for supply of the plastic material. The output of the extruder is connected to the extruder head and on its discharge nozzle which can preferably be regulated in opening width an extruded tube or one extruded in several layers emerges. The extruded tube is transferred to a blow molding tool and inflated within its cavity with a blowing mandrel. The plastic tube can have one or more layers, it can be extruded as a tube with visual strips, decorative strips, or, relative to the periphery, with several segments for example of different colors.
The blowing station with the blowing mandrel is conventionally located laterally from the extrusion head and the blow molding tool which is supplied with the extruded tube must be moved into the blowing station where then the blowing mandrel is conventionally transported from overhead into the blow molding cavity. For continuous operation in one type of known extrusion blowing machines, there are conventionally two blowing stations. Each blowing station is equipped with one blow molding tool. The blowing stations are located opposite one another on either side of the extruder and have blow molding tables with the blow molding tools which are moved alternately under the extruder head in order to receive the extruded tube. In doing so the blow molding tool is opened for retrieving the tube. After closing the blow molding tool the tube is cut off between the extrusion head and the blow molding tool. Afterwards the blow molding table is moved again into the blowing station, where the blowing mandrel is transported into the cavity of the blow molding tool and the tube is inflated according to the blow molding cavity and afterwards removed. For multiple extrusion heads and multiple blow molding tools each blowing station is equipped with a corresponding number of blowing mandrels which are jointly transported into the blow molding cavities. Altogether the extruder with the extruder head and the two blow molding stations form roughly the shape of a T. The extruder with the extruder head constitutes the long leg of the T, while the two blowing tables can be moved alternately along the short crosspiece halves to under the extruder head.
Extrusion blowing machines of the above described type have been tested and allow high production performance. But there is still a desire for improvements in order to further reduce the required tool investments, i.e. the costs per blow molding cavity. The necessity of moving the blow molding tables with the blow molding tools laterally to the blowing stations leads to dead times which result from the path to be traversed and the speed of motion of the blow molding tables. Due to the relatively great masses which must be accelerated and braked again, the speed of motion can not be selected to be optionally large. Also the paths to be traversed laterally cannot be further shortened for construction reasons. The extruded tube must be cut off after the blow molding tool is filled. This conventionally takes place by a laterally supplied cutting blade. The tube part which continues to be extruded and which projects from the extrusion nozzle swings back and forth after the cutting process. In particular, for very high clock cycles the swinging of the tube can lead to problems in the transfer of the tube to the blow molding cavity.
Extrusion blowing machines are known in which a number of blow molding tools are located on a rotating wheel. The wheel stands roughly vertically and guides the blow molding tools roughly tangentially to the plastic tube which has been continuously extruded by the extrusion head. Shortly before reaching the extruded plastic tube, the guided blow molding tool is opened to retrieve the tube. As the wheel continues to turn, the blow molding tool is closed around the inserted tube and the latter is finally cut off during further turning. The arrangement of the blow molding tools and the speed of rotation of the wheel are chosen such that the tube is only cut off when the following blow molding tool has closed around the next tube piece. The tube which is located in the cavity of the blow molding tool as the wheel continues to turn finally travels into the blowing station where it is inflated by way of a blowing mandrel which is inserted laterally into the mold tool according to the blow molding cavity. Finally, the inflated hollow body is discharged from the blow molding tool by opening. The blow molding tool which is located on the rotating wheel is closed again as motion continues and is moved again to the extrusion head in order to accommodate another extruded tube.
The disadvantage in the wheel blowing machines is the circumstance that due to the large number of separate blow molding tools located on the wheel, they require a relatively high investment cost for preparation of the blow molding tools. In general the blow molding tools are not completely identical. This can lead to quality differences in the manufactured hollow bodies from blow molding tool to blow molding tool. The use of blow molding tools with several blow molding cavities is relatively difficult and expensive. The blow molding tools can only be attached at fixed mounting points on the wheel. They are fixed according to the height of the blown hollow body which can be produced at maximum with the machine. The mounting points, once established, can no longer be changed. This is also a result of the mechanical controls which are conventional in these machines via control cams, cam disks and the like. If containers with a smaller height are to be blown on the wheel blowing machine, the smaller blow molding tools mounted on the periphery of the wheel have a greater angular distance from one another. Since the plastic tube emerges continuously from the extrusion head, this leads to larger amounts of scrap in the areas between the two blow molding tools. The extruded plastic tube is accepted by the blow molding tool which has moved past along a circular shape under the extrusion head. Arc motion is superimposed on the lateral feed motion of the closing blow mold halves. By a radius of the wheel as large as possible, the attempt is made to keep this arc motion of the blow mold halves as small as possible when the plastic tube is being accepted; but it cannot be completely eliminated. The circumstance that the tube between two successive blow molding tools on the one hand is held by the extruder head and on the other hand by the advancing blow molding tool, cannot change anything in the geometrical relationships. Under certain circumstances therefore for more complex container geometries it can occur that the tube is not ideally inserted into the blowing cavity and is partially pinched between the adjoining areas of the closing blow mold halves. This can lead to unwanted scrap.